1. Field of the Invention
The present invention generally relates to a sealing material, in particular, to a lead-free sealing material.
2. Description of Related Art
Low-temperature sealing glass materials have been used in display, microelectronics, semiconductor, and other fields to bond the elements, for example, for the glass-to-glass, glass-to-metal, glass-to-ceramics, ceramics-to-ceramics, and metal-to-ceramics bond.
The characteristics of the low-temperature sealing glass materials are as follows.
1. The glass softening temperature must be low enough (usually lower than 500° C.), so as to prevent the element to be bonded from being damaged during the bonding process, as the element is not high temperature resistant.
2. The thermal expansion coefficients of the glass and the element to be bonded must be as close to each other as possible, so as to prevent the structural twisting deformation or function loss due to an internal stress generated during a cooling process after the bonding process.
3. The viscosity of the glass must be appropriate, and the glass and the element to be bonded have a good wettability during the bonding process.
4. The glass has good chemical durability and weather resistance.
5. The glass should not react with the element to be bonded significantly during the bonding process, so as to prevent the element from being damaged.
Previously, most of the composition of the low-temperature sealing glass contains a great amount of lead, but lead and the compound thereof are hypertoxic and are difficult to be decomposed, and thus being extremely harmful to human body and environment. From July of 2006, European Union began to comprehensively implement “Restriction on the Use of Hazardous Substances (RoHS)”, in which materials containing lead, cadmium, mercury, and hexavalent chromium (Cr6+) are not permitted to be applied to any electronic products, and the entry of the lead containing solder products to the European Union will be completely prohibited in 2008. Therefore, under the situation that the environmental protection consciousness is increasing, and in order to satisfy the environmental protection regulations of the European Union, it is extremely important to develop a new lead-free sealing glass material with a low melting temperature.
Recently, although the lead-free sealing glass material of V2O3—ZnO—BaO—P2O5 or Bi2O3—SiO2—B2O3 system is free of lead, the glass softening temperature is quite high, so it is not used in the low-temperature field. Further, CuO—P2O5 system is also used, and alkali metals and alkali earth oxides are added to prepare the lead-free sealing glass with the low melting temperature, in which the glass transition temperature is approximately 400-500° C. However, the alkali metals and the copper ion may affect the semiconductor function, so it may have problems in application.